The present disclosure relates generally to semiconductor devices, particularly to field-effect semiconductor devices as typified by high electron mobility transistors (HEMT), and particularly to such devices that operate normally off. The present disclosure also provides methods of fabricating such normally-off field-effect semiconductor devices.
Gallium nitride (GaN) semiconductor devices are increasingly desirable for power semiconductor devices because of their ability to carry large current and support high voltages. Development of these devices has generally been aimed at high power/high frequency applications. Devices fabricated for these types of applications are based on general device structures that exhibit high electron mobility and are referred to as heterojunction field effect transistors (HFET) or high electron mobility transistor (HEMT).
A GaN HEMT includes a nitride semiconductor with at least two nitride layers of different nitride materials, formed on a substrate or a buffer, while source, drain and gate electrodes are disposed in prescribed positions on the nitride semiconductor. The difference between the two nitride materials causes the layers to have different band gaps, and form a heterojunction therebetween. Spontaneous and piezoelectric depolarizations of this heretojunction contribute to a conductive two dimensional electron gas (2DEG) region near the heretojunction, specifically in the nitride layer with the narrower band gap.
Because the 2DEG region exists under the gate at zero bias, most GaN HEMT devices are normally on, or depletion mode devices. If the 2DEG region is depleted, i.e. removed, below the gate at zero bias, the device can be an enhancement mode device. Enhancement mode devices are normally off, and are more desirable because they are easier to control with simple, low-cost driver. A normally-off device, or an enhancement-mode device, requires a positive gate bias in order to conduct current.
Attempts have been made to fabricate a normally-off HEMT, by, for example, making the nitride layer with the wider band gap thinner only under the gate electrode. By doing so, the electric field due to the depolarizations caused by the heterojunction under the gate electrode weakens, resulting in diminished electron concentration in the 2DEG region. The 2DEG region disappears just under the gate electrode, and the HEMT can be held off between drain and source electrodes while no bias voltage is applied to the gate electrode. Selective etching is normally employed to make the nitride layer thinner. The selective etching, however, likely leads to impairment of the semiconductor crystal structures of the nitride layers, and the resulting HEMTs are not necessarily satisfactory in performance.
P-type nitride material has been used to relatively raise the bands nearby the heterojunction, so the 2DEG disappears. Nevertheless, this attempt for fabricating a normally-off HEMT requires extra photo masks to define the pattern of the p-type nitride material and an associated gate electrode. These extra photo masks and related etching processes lead to wider cell pitch and higher RdsON (“ON resistance”), and also increase manufacturing costs. Furthermore, this kind of method for fabricating a normally-off HEMT needs an etching process to remove the P-type nitride material outside gate regions where the gate electrodes of normally-off HEMTs are located. The end point of this etching process is hard to control to stop before damaging active regions, however. Once over etched, active regions for normally-on HEMTs will be damaged, resulting in diminished electron concentration in the 2DEG regions therein and degraded performance in conductivity.